internal/mcp: use RawMessage for tool args

Change the type of CallToolParams.Arguments to json.RawMessage.

Change-Id: Ief16ef261b9f41566a1a16efc60a9382a8d414eb
Reviewed-on: https://go-review.googlesource.com/c/tools/+/672615
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Findley <rfindley@google.com>

mcp/generate.go

@@ -53,8 +53,15 @@ type config map[string]*typeConfig
 var declarations = config{
 	"Annotations": {},
 	"CallToolRequest": {
-		Name:   "-",
-		Fields: config{"Params": {Name: "CallToolParams"}},
+		Name: "-",
+		Fields: config{
+			"Params": {
+				Name: "CallToolParams",
+				Fields: config{
+					"Arguments": {Substitute: "json.RawMessage"},
+				},
+			},
+		},
 	},
 	"CallToolResult": {},
 	"CancelledNotification": {